The present invention relates, in general, to reading data from and restoring data in a data storage element, and more particularly, to reading data from and restoring data in a ferroelectric random access memory (FERAM) cell.
The packing density and the power consumption of nonvolatile data storage elements are becoming increasingly important issues in low power applications, including portable computers and personal communication. Typically, a ferroelectric nonvolatile data storage element includes a capacitor with polarization retention for storing information and a switch, such as a bi-directional pass gate, for accessing the capacitor. When a bi-directional pass gate, e.g., a complementary metal oxide semiconductor (CMOS) pass gate, is used as a switch to access a ferroelectric memory capacitor, data can be stored and restored without degradation because there is no voltage drop across the pass gate. However, in high density FERAM designs, a pass gate having a single transistor is preferable because it uses less silicon area than a bi-directional pass gate.
As those skilled in the art are aware, a high level voltage transmitted through a single n-channel insulated gate field effect transistor pass gate is degraded by an amount equal to one transistor threshold voltage, which results in a voltage degradation when storing and restoring a high voltage representing a logical "one" in a FERAM cell. Similarly, a low level voltage transmitted through a single p-channel insulated gate field effect transistor pass gate is degraded by an amount equal to one transistor threshold voltage, which results in a voltage degradation when storing and restoring a low voltage representing a logical "zero" in a FERAM cell. As a result, the memory cell does not operate symmetrically and less charge is extracted from the memory cell during subsequent "read" operations.
One technique for preventing data degradation in a FEBAM having a single transistor pass gate is to use a word line booster. This technique uses a booster to apply a word line voltage, which is one threshold voltage higher than the supply voltage, to the gate electrode of the pass gate transistor. The higher gate voltage prevents the voltage degradation. However, each time the word line voltage is boosted, the booster consumes power. Furthermore, boosting the word line voltage requires additional circuitry that takes silicon area.
Accordingly, it would be advantageous to have an energy efficient technique for reading and fully restoring data in a nonvolatile memory cell that has a single transistor pass gate. It would be of further advantage for the technique to be implemented without using additional circuitry.